In general, a rotary wing aircraft is piloted by adjusting the cyclic pitch and the collective pitch of at least one main rotor for providing lift and forward advance, and by adjusting operating parameters of a tail rotor (in particular its collective pitch).
More generally, piloting takes place along pilot axes: roll axis, tilt (pitch) axis, yaw axis, and “collective” axis corresponding to adjusting the collective pitch of the blades of the main rotor.
These adjustments are performed by actuators controlled by a computer; the instructions for controlling actuators as issued by the computer of the autopilot (referred to below as “AP”), are generated on the basis of setpoint values selected by the pilot as a function of the stage of flight to be undertaken, and as a function of values for state parameters of the aircraft, such as its altitude or its air speed, as delivered by on-board sensors.
For this purpose, the computer is programmed to apply servo-control relationships enabling the state parameters of the aircraft to be servo-controlled as a function of the setpoints—or targets—selected by the pilot of the aircraft.
The AP may include one or more target relationships for a given pilot axis.
In particular, for piloting about the tilt axis, the AP may have a vertical target first relationship seeking to cause a state parameter of the helicopter to reach and maintain a setpoint for this parameter, with this relationship being used to issue an instruction to cause the main rotor to tilt; the AP described in patents FR 2 830 631 and U.S. Pat. No. 6,691,950 also including, for the tilt axis, a second relationship seeking to limit air speed relative to two limit values (maximum and minimum authorized air speeds); in order to switch from one relationship to another as a function of the actual air speed of the helicopter, the AP includes a set of comparators and selectors; that configuration makes it possible to manage longitudinal or tilt control of the main rotor automatically.
For piloting the collective axis, the AP described in that document further includes a vertical target relationship and a relationship for maintaining the power at the maximum authorized value, together with a selector to select the power relationship when the air speed reaches the maximum value compatible with maintaining the vertical target and with the maximum authorized power.
The AP described in patent applications FR 2 830 630 and US 2003 066927 has two target relationships for the tilt axis and two target relationships for the collective axis; first selector means select a vertical target relationship for the collective axis during normal operation of the helicopter and switch, for this axis, to a relationship for maintaining power when the power becomes insufficient to maintain an air speed setpoint; second selector means select a forward-advance target (or setpoint) relationship for the tilt axis when the vertical target relationship is selected for the collective axis, and switches, for the tilt axis, to a vertical target relationship when the first selector means select the power-maintaining relationship.
In autopilot systems for rotary wing aircraft that provide for switching from a first relationship to a second relationship for controlling an axis, the switchover can give rise to troublesome jumps, beats, or oscillations.
An object of the invention is to remedy that drawback.
An object of the invention is to propose an autopilot system for a rotary wing aircraft that is improved and/or that remedies, at least in part, the drawbacks of known systems for automatically piloting such aircraft.